Electropolishing process

ABSTRACT

The present invention relates to a method for the electropolishing of surfaces of metals and metal alloys. Said method is characterized in particular in that it can be applied to a wide range of metals. Thus, it is suitable for the electropolishing of metal surfaces comprising iron, tungsten, magnesium, aluminum or an alloy of these metals. The electrolyte used in the method comprises methanesulfonic acid and at least one alcoholic compound selected from aliphatic diols of general formula C n H 2n (OH) 2  with n=2-6 and alicyclic alcohols of general formula C m H 2m-1 OH with m=5-8.

FIELD OF THE INVENTION

The present invention relates to a method for the electrochemicalpolishing of surfaces of metals and metal alloys.

BACKGROUND OF THE INVENTION

The purpose of the process of electrochemical polishing orelectropolishing is to produce metal surfaces of high purity, and tosmooth and deburr the metal surfaces. Smoothing in the micro-range canalso produce gloss of the surfaces so treated. Furthermore,electropolishing can also remove any stresses from the outer layers ofthe material.

There are a great many different electropolishing processes that can beused for the processing of various metals or metal alloys. As a rulethese processes are based on the use of electrolytes that comprise aconcentrated inorganic acid such as phosphoric acid or sulfuric acid ora mixture of concentrated inorganic acids, often with additives forfurther enhancing the action of the electrolytes so as to obtainsmoother and shinier metal surfaces. Some examples of such additives arechromic acid, hydrofluoric acid, amine fluorides or organic additives,such as alcohols, amines, glycerol, etc.

However, a feature that is common to all of these existing electrolytes,which are widely used industrially, is that they can only be usedsuccessfully for certain metals and/or alloys and thus have a veryrestricted application profile. For the processing of various metals oralloys it is therefore often necessary to maintain a correspondingnumber of different electrolytes. Often the individual electrolytes mustbe kept strictly separate and in particular must not be mixed, as anymixing may damage them and make them unusable. Sometimes this can evenresult in certain constituents of the electrolytes reacting togetherand, for example, releasing substances that are hazardous or injuriousto health. Furthermore, the requirements on execution of the process andequipment of the electropolishing installations are also often veryvaried on account of the different electrolytes, so that severalinstallations have to be maintained for different materials.

The electrolytes usually employed are often hazardous materials, whichdepending on their particular toxicity, flammability and/or danger classare subject to special requirements and regulations with respect totheir storage and use and require appropriate precautions in connectionwith environmental protection and labor safety. This in turn causesconsiderable expenditure and the associated costs.

The ideal solution for these problems would be an electropolishingprocess that is equally suitable for the processing of all metals andmetal alloys and is largely harmless with regard to the associatedenvironmental impact and labor safety.

An electrolyte that largely meets the requirements for universalapplication has long been known from the state of the art. This is amixture of perchloric acid and acetic anhydride. Often, however, thismixture cannot be used industrially owing to the associated explosionrisk, or can only be used with considerable expenditure on safetymeasures.

Patent application WO 01/71068 A1 discloses electrolytic polishingprocesses that can apparently be used for a wide range of metals ormetal alloys. These electropolishing processes use, among other things,an electrolyte of methanesulfonic acid and methanol. This electrolytehas, however, the serious drawback that because of its high proportionof more than 80% of highly volatile methanol it is a health hazard, andpresents a risk of fire and explosion. Therefore such a process can as arule only be used at very low temperatures, for example max. 10° C., orwith an expensive system for trapping the resultant vapors and carryingthem away. Furthermore, suitability of this process for carbon steels,magnesium, magnesium alloys or aluminum-silicon alloys has not beendisclosed.

The method disclosed in patent application U.S. 2005/0045491 A1 alsoappears to have a relatively wide range of applications, but once again,suitability for magnesium-containing metal surfaces or those comprisingaluminum-silicon alloys is not disclosed. The electrolyte used containsat least 75 wt. % of an alkylene glycol, the remainder comprising achloride salt of alkali metals and/or alkaline-earth metals.

SUMMARY OF THE INVENTION

The present invention relates to an electropolishing process that can beused successfully for a wide range of metals and metal alloys and issubstantially harmless with respect to labor safety and environmentalprotection. The electrolyte used for this comprises methanesulfonic acidand at least one alcoholic compound, which is selected from aliphaticdiols and alicyclic alcohols.

The method is suitable for the electropolishing of surfaces of suchdiverse metals as iron, tungsten and light metals, and of surfaces ofalloys of these metals. In particular it is suitable for surfaces ofiron or an iron alloy, such as ferronickel, special steel (stainlesssteels) or carbon steel, which can be electropolished according to thepresent process both in the hardened and in the unhardened form; oftungsten or a tungsten alloy, of magnesium, a magnesium alloy, aluminumor an aluminum alloy, as well as an aluminum-silicon alloy. An alloy ofa particular metal means alloys in which this metal is the mainconstituent of the alloy, based on the weight of the constituents of thealloy. Often said metal (or metals) comprises more than 50 wt. % of thealloy.

The electrolyte used in the electropolishing process according to thepresent invention is a solution comprising methanesulfonic acid and atleast one alcoholic compound, the at least one alcoholic compound beingselected from the group comprising aliphatic diols of general formulaC_(n)H_(2n)(OH)₂ with n=2-6 and alicyclic alcohols of general formulaC_(m)H_(2m-1)OH with m=5-8. In particular, the alcoholic compound cancomprise at least one aliphatic diol of general formulaC_(n)H_(2n)(OH)₂, with n=3, 4, 5 or 6. Moreover, all isomers of thesealiphatic diols can be used, provided the two hydroxyl groups are boundto different carbon atoms. Examples that may be mentioned are thecompounds 1,2-propanediol, 1,2-butanediol or 1,4-butanediol.

In a special embodiment, the electrolyte contains, as alcoholiccompounds, both at least one aliphatic diol of general formulaC_(n)H_(2n)(OH)₂ and at least one alicyclic alcohol of general formulaC_(m)H_(2m-1)OH, where n=2-6 and m=5-8.

The alicyclic alcohols of the present invention also comprise allisomers satisfying the general formula C_(m)H_(2m-1)OH with m=5-8. Allcarbon atoms can form the ring structure, such as in cyclopentanol,cyclohexanol, cycloheptanol and cyclooctanol; it is also possible,however, for one or more carbon atoms to form a hydroxyalkyl and/or oneor more alkyl side chain(s). Electrolyte solutions comprisingcyclohexanol are especially preferred.

In a preferred embodiment, the electrolyte according to the method forelectropolishing of the present invention comprises a mixture comprising5-93% methanesulfonic acid and 95-7% of the at least one alcoholiccompound. These percentages and all others in the present applicationrelate, unless stated otherwise, to the weight of the respectivesubstances and solutions. It is especially preferred for the electrolyteto comprise 10-80% methanesulfonic acid and 90-20% of the at least onealcoholic compound. Thus, the electrolyte can comprise for example20-50% methanesulfonic acid and 50-80% of the at least one alcoholiccompound.

In particular, the method for electropolishing according to the presentinvention is characterized in that apart from methanesulfonic acid andalcoholic compounds, no other additives are required for theelectrolyte. It should be mentioned in particular that the electrolyteused in this method contains neither chromic acid or chromates, norperchloric acid or its salts. Moreover, the method does not use anyhighly volatile additives such as methanol, ethanol or esters, the highvapor pressure of which presents a particular challenge for labor safetyboth with respect to their flammability and their toxicity. In addition,the electrolyte does not contain any hydrofluoric acid and also on thisbasis is largely problem-free in operation.

Preferably, the electrolyte used in the method according to the presentinvention contains no water or only small amounts of water. Thus, thewater content of the electrolyte should not exceed a proportion of 10%water. Furthermore, the electrolyte does not require any addition ofsalts to increase its conductivity.

In a special embodiment of the present invention the method is carriedout at a temperature between 40° C. and 100° C. It is especiallypreferred for the method to be carried out at a temperature between 60°C. and 100° C. Since the electrolyte of the present method does notcontain any highly volatile constituents, higher temperatures can beused, for example temperatures up to 80° C., up to 90° C., up to 100° C.or even higher, without the need for special precautions, for examplefor reliable capture and removal of vapors that form. The possibilitythat the method can also be carried out at higher temperatures makes itpossible, on the one hand, for the electropolishing process to becarried out if necessary in a relatively short time, and on the otherhand it means that expensive removal of the heat released in theelectropolishing process becomes unnecessary. Accordingly, expensivecooling becomes largely or completely unnecessary. If cooling is used,it therefore does not have to satisfy high performance requirements.

There is also considerable freedom in the choice of anodic currentdensity in the method presented here. Depending on the particular metal,values between 3 and 40 A/dm² of the surface to be polished arepreferred, and values in the range 5-30 A/dm² are especially preferred.Tungsten or tungsten alloys in particular permit the use of higheranodic current densities of for example about 30-40 A/dm². However, theother materials described here can also be electropolished successfullyat higher anodic current densities. For surfaces containing iron,aluminum and magnesium, however, anodic current densities of about 5-20A/dm² are generally entirely sufficient.

The duration of the electropolishing operation depends of course on themetal being processed, the roughness of the workpiece to be polished,the desired amount of metal removal and the desired smoothing of theworkpiece surfaces, and the temperature and the current density.

In addition to its wide applications, the method according to theinvention possesses other important advantages over the existingelectropolishing processes. Thus, the electrolyte used is not chemicallyaggressive and therefore after switching off the electropolishingcurrent, as well as during the subsequent rinsing operations, it remainssubstantially inert with respect to the surfaces being electropolished.The surfaces are not chemically attacked and etched, so that the qualityof the electropolished surfaces is maintained and no special measuresare required for removing the electrolyte as quickly as possible fromthe treated workpiece. This is particularly important in the processingof workpieces with low corrosion resistance, for instance ordinarysteel, magnesium, aluminum and their alloys.

In addition to the method itself in all its aspects presented here, afurther aspect of the present invention relates to the electrolytesdescribed above that are used in this method.

The invention is explained in more detail in the following examples.These examples only represent possible embodiments of theelectropolishing process described here and of the electrolytes usedtherein, and do not in any way imply a restriction to the conditionsused here.

EXAMPLES 1

Treated surface: Special steel, Material No. 1.4301

Electrolyte: 37% methanesulfonic acid+63% 1,2-propanediol Temperature:80° C.

Anodic current density: 10 A/dm²

Duration: 15 min Result: Mirror finish 2

Treated surface: tool steel (carbon steel)

Electrolyte: 37% methanesulfonic acid+63% 1,2-pentanediol Temperature:80° C. Anodic current density; 20 A/dm² Duration: 10 min Result: highlypolished 3

Treated surface: tungsten

Electrolyte: 50% methanesulfonic acid+50% 1,2-propanediol Temperature:80° C.

Anodic current density: 40 A/dm²

Result: highly polished 4

Treated surface: magnesium

Electrolyte: 20% methanesulfonic acid+40% 1,2-propanediol+40%cyclohexanol Temperature: 60° C.

Anodic current density: 10 A/dm²

Duration: 8 minutes Result: highly polished 5

Treated surface: aluminum-silicon alloy AlSi₂O

Electrolyte: 20% methanesulfonic acid+80% 1,2-butanediol Temperature:80° C. Anodic current density; 10 A/dm² Duration: 12 min Result: highlypolished 6

Treated surface: aluminum-magnesium alloy AlMg₁

Electrolyte: 50% methanesulfonic acid+50% 1,2-propanediol Temperature:80° C.

Anodic current density: 10 A/dm²

Duration: 10 min

Result: highly polished

What is claimed is:
 1. A method for the electropolishing of surfaces ofmetals, comprising exposing a metal surface to an electrolyte in thepresence of an electric current, wherein the metal surface is selectedfrom iron, tungsten, a light metal, and an alloy thereof, and whereinthe electrolyte comprises: methanesulfonic acid, and at least onealcoholic compound selected from the group comprising aliphatic diols ofgeneral formula C_(n)H_(2n)(OH)₂ with n=2-6 and alicyclic alcohols ofgeneral formula C_(m)H_(2m-1)OH with m=5-8.
 2. The method as claimed inclaim 1, wherein the surface comprises iron or an iron alloy, hardenedor unhardened.
 3. The method according to claim 2, wherein the ironalloy is ferro-nickel, special steel, or carbon steel.
 4. The method asclaimed in claim 1, wherein the surface comprises tungsten or a tungstenalloy.
 5. The method as claimed in claim 1, wherein the surfacecomprises magnesium, a magnesium alloy, aluminium, or an aluminum alloy.6. The method as claimed in claim 1, wherein the surface comprises analuminum-silicon alloy.
 7. The method as claimed in claim 1, wherein thealcoholic compound comprises at least one aliphatic diol of generalformula C_(n)H_(2n)(OH)₂ with n=3-6.
 8. The method as claimed in claim7, wherein the aliphatic diol comprises 1,2-propanediol and/or1,2-butanediol.
 9. The method as claimed in claim 1, wherein theelectrolyte comprises 5 to 93 wt. % methanesulfonic acid and 95 to 7 wt.% of the alcoholic compound or compounds.
 10. The method as claimed inclaim 1, wherein the electrolyte comprises 10 to 80 wt. %methanesulfonic acid and 90 to 20 wt. % of the alcoholic compound orcompounds.
 11. The method as claimed in claim 1, wherein the alcoholiccompound comprises at least one aliphatic diol and at least onealicyclic alcohol.
 12. The method as claimed in claim 1, wherein thealicyclic alcohol comprises cyclohexanol.
 13. The method as claimed inclaim 1, wherein the electrolyte does not contain any chromic acid orchromates.
 14. The method as claimed in claim 1, wherein the method iscarried out at a temperature between 40 and 100° C.
 15. The method asclaimed in claim 1, wherein the method is carried out at a temperaturebetween 60 and 100° C.
 16. The method as claimed in claim 1, wherein themethod is carried out at an anodic current density of 3 to 40 A/d m².17. An electrolyte for the electropolishing of surfaces of metals, whichare selected from iron, tungsten and light metals, and from alloys ofthese metals, wherein the electrolyte comprises methanesulfonic acid andat least one alcoholic compound consisting of an aliphatic diol ofgeneral formula C_(n)H_(2n)(OH)₂ with n=2-6 or an alicyclic alcohol ofgeneral formula C_(m)H_(2m-1)OH with m=5-8.
 18. The electrolyte asclaimed in claim 17, wherein the alcoholic compound comprises at leastone aliphatic diol of general formula C_(n)H_(2n)(OH)₂ with n=3-6. 19.The electrolyte as claimed in claim 17, wherein the aliphatic diolcomprises 1,2-propanediol and/or 1,2-butanediol.
 20. The electrolyte asclaimed in claim 17, wherein the electrolyte comprises 5 to 93 wt. %methanesulfonic acid and 95 to 7 wt. % of the alcoholic compound orcompounds.
 21. The electrolyte as claimed in claim 17, wherein theelectrolyte comprises 10 to 80 wt. % methanesulfonic acid and 90 to 20wt. % of the alcoholic compound or compounds.
 22. The electrolyte asclaimed in claim 17, wherein the alcoholic compound comprises at leastone aliphatic diol and at least one alicyclic alcohol.
 23. Theelectrolyte as claimed in claim 17, wherein the alicyclic alcoholcomprises cyclohexanol.